Still Life Example Project

Ambient Occlusion and Using Overlay Mode in Photoshop

This page is designed as a supplement to lectures. It is not intended to be a stand alone tutorial.

Ambient Occlusion is an indirect lighting method where the illumination of each sample point in the scene is based on the other geometry in the scene. If there is little geometry near the sample point the value is light. If there is geometry near the point, its value will be dark. This effect creates a very soft shadow look, but more importantly it emphasizes small details evenly through the scene. An Ambient Occlusion render is typically 256 shades of gray.

Many times an Ambient Occlusion render can be beautiful on its own, but usually it is combined with a more straight forward render that includes all the textures, lighting effects, and shadow.

This is the rendered image using 5 lights to create a dramatic effect. However, upon close inspection it is clear that many of the details of the objects are not clear or too dark. Also, the objects just don't appear to be grounded - sitting on the ground. Achieving a more realistic look is difficult (if not impossible) to achieve using just direct lighting methods.

This is an Ambient Occlusion render of the same scene and from the same camera angle. Note how rich the detail is and how objects are very "attached" to the ground they are sitting on. There is detail in all areas of the scene.

To do an Ambient Occlusion render in Maya:

Delete or turn off all lights.

In Render Globals>Common>RenderOptions, turn off "Enable Default Lights".

Switch to Mental Ray Rendering. Increase the quality to "Production".

Select the camera and change to Environment>BackgroundColor to what you want: Black, White, or Medium (50%) Gray.

Create a Surface Shader and assign it to ALL objects.

In the HyperShade under MentalRay>Textures create a "mib_amb_occlusion" node. Simply drag this node to the surface shader and connect it using "Default"

Render the scene. With the default settings it will be very spotty

Open the mib_amb_occlusion node in the Attribute Editor. Increase the sample up to 32, 64, or 128. This will increase the render time, but will smooth out the results.

If you want to combine (composite) the Color Pass Render with the Ambient Occlusion Pass, you can do this quickly in Photoshop, AfterEffects or other compositing type programs. This example here uses PhotoShop.

Both images need to rendered the same size and from the same angle and same camera. They have to match exactly.

Paste the Ambient Occlusion render in a layer above the Color Pass

Change the Blending Mode to "Overlay". Overlay basically lightens and darkens the lower layers based on the level of gray. A medium gray will not alter the lower layer. Brighter grays will lighten and darker grays will darken.

You can change the layers opacity to soften its effect.

You can also apply a Levels Adjustment t the layer to change its contrast, brightness, and darkness.

Here is the result. Though it is hard to see on this web-page layout, when compared to the original render, it is much more detailed and perhaps realistic.

UMBC Department of Visual Arts, Advanced Maya Courses, Dan Bailey

This is the rendered image using 5 lights to create a dramatic effect. However, upon close inspection it is clear that many of the details of the objects are not clear or too dark. Also, the objects just don't appear to be grounded - sitting on the ground. Achieving a more realistic look is difficult (if not impossible) to achieve using just direct lighting methods.
This is an Ambient Occlusion render of the same scene and from the same camera angle. Note how rich the detail is and how objects are very "attached" to the ground they are sitting on. There is detail in all areas of the scene. To do an Ambient Occlusion render in Maya: Delete or turn off all lights. In Render Globals>Common>RenderOptions, turn off "Enable Default Lights". Switch to Mental Ray Rendering. Increase the quality to "Production". Select the camera and change to Environment>BackgroundColor to what you want: Black, White, or Medium (50%) Gray. Create a Surface Shader and assign it to ALL objects. In the HyperShade under MentalRay>Textures create a "mib_amb_occlusion" node. Simply drag this node to the surface shader and connect it using "Default" Render the scene. With the default settings it will be very spotty Open the mib_amb_occlusion node in the Attribute Editor. Increase the sample up to 32, 64, or 128. This will increase the render time, but will smooth out the results.
If you want to combine (composite) the Color Pass Render with the Ambient Occlusion Pass, you can do this quickly in Photoshop, AfterEffects or other compositing type programs. This example here uses PhotoShop. Both images need to rendered the same size and from the same angle and same camera. They have to match exactly. Paste the Ambient Occlusion render in a layer above the Color Pass Change the Blending Mode to "Overlay". Overlay basically lightens and darkens the lower layers based on the level of gray. A medium gray will not alter the lower layer. Brighter grays will lighten and darker grays will darken. You can change the layers opacity to soften its effect. You can also apply a Levels Adjustment t the layer to change its contrast, brightness, and darkness.
Here is the result. Though it is hard to see on this web-page layout, when compared to the original render, it is much more detailed and perhaps realistic.